2023 Hypoxia Season Quick Facts

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Background

Hypoxia is a condition of low dissolved oxygen (DO) concentrations in the waters of Long Island Sound (LIS) that impacts up to half of the Sound’s waters each summer. The primary cause of hypoxia is nitrogen enrichment that comes from a variety of sources throughout the Long Island Sound watershed, in particular sewage treatment plants, stormwater runoff and atmospheric deposition.

For LIS, DO levels below 3.0 mg/L are considered hypoxic, causing mobile animals to leave and sessile animals to die or be physically or behaviorally impaired. However, early studies in LIS by Connecticut Department of Energy and Environmental Protection (CT DEEP) Marine Fisheries biologists found that DO can become limiting below 4.8 mg/L for sensitive fish species, while more tolerant species are not affected until DO falls below 2.0 mg/L (Simpson et al, 1995, 1996).

Since 1991, CT DEEP and the Interstate Environmental Commission (IEC) have conducted an intensive water quality monitoring program on Long Island Sound. The program is funded through a grant from the EPA’s Long Island Sound Study. Data from the surveys are used to quantify and identify annual trends and differences in water quality parameters relevant to hypoxia (low dissolved oxygen), especially nutrients, temperature, and chlorophyll. These data are also used to evaluate the effectiveness of the management program to reduce nitrogen concentrations, since nitrogen is a primary contributor to the excessive algae growth that leads to hypoxia in LIS.

During the summer (June -September), surveys are conducted at bi-weekly intervals to better define the areal extent and duration of hypoxia. During these surveys stations are sampled for in situ parameters including dissolved oxygen, temperature, pH, and salinity.

The table below provides a summary of the 2023 Hypoxia Season at a glance.

During the summer of 2023, CT DEEP conducted seven surveys between May 31 and September 7, while IEC conducted twelve surveys between June 28 and September 12. Hypoxic conditions were documented during three CT DEEP surveys and nine IEC surveys.

Data from all CTDEEP cruises can be downloaded from the UCONN ERDDAP. All samples are collected and analyzed under EPA-approved Quality Assurance Project Plans.

2023 Long Island Sound Hypoxia Season Cruise Information
Cruise Start Date End Date Number of Stations Sampled Number of Hypoxic Stations Hypoxic Area (mi2) Minimum DO Station Where Minimum DO Occurred
WQJUN23 05-31-2023 06-02-2023 17 0 0 6.24 B3
HYJUN23 06-23-2023 06-23-2023 18 0 0 6.38 A4
IEC RUN #1 06-28-2023 06-28-2023 22 0 NC 4.22 H-C1
WQJUL23 07-05-2023 07-10-2023 41 0 0 3.41 A4
IEC RUN #2 07-05-2023 07-05-2023 22 4 NC 1.87 9-413
IEC RUN #3 07-12-2023 07-12-2023 22 4 NC 2.25 H-C1
HYJUL23 07-17-2023 07-19-2023 40 3 42.01 1.85 A4
IEC RUN #4 07-18-2023 07-18-2023 22 12 NC 1.30 9-413
IEC RUN #5 07-25-2023 07-25-2023 22 6 NC 0.63 A5
WQAUG23 07-31-2023 08-03-2023 42 13 126.77 1.69 A4
IEC RUN #6 08-03-2023 08-03-2023 21 8 NC 0.97 H-D
IEC RUN #7 08-09-2023 08-09-2023 22 4 NC 2.57 H-B
IEC RUN #8 08-15-2023 08-15-2023 22 12 NC 1.37 B4
HYAUG23 08-14-2023 08-16-2023 38 8 136.72 1.94 A4
IEC RUN #9 08-22-2023 08-22-2023 22 0 NC 3.17 A3
IEC RUN #10 08-31-2023 08-31-2023 21 4 NC 2.23 H-B
WQSEP23 08-29-2023 09-07-2023 42 0 0 3.56 A4
IEC RUN #11 09-06-2023 09-06-2023 21 0 NC 4.14 H-C
HYSEP23 No Survey Conducted- DO > 3.0 mg/L NA NA NA NA NA NA
IEC RUN #12 09-12-2023 09-12-2023 22 14 NC 1.17 9-409

Weather Summary

Leading up to 2023’s hypoxia survey season, Connecticut experienced its warmest winter on record, with temperatures 2 F to 8 F above normal in February. Much of the Northeast ranked within the warmest 20 for maximum temperature averages during the Spring season (Mar-May).

Precipitation ramped up in June, causing major flooding across the Northeast and increasing the flow of debris and runoff into the region’s rivers. June also had a drop in temperatures and air quality, with winds funneling smoke from Quebec wildfires into the region.

Moving into July, there were both record heat and rainfall. Precipitation fell at 200% to 300% of normal, making it the fourth wettest July on record for the Northeast. Twenty-three major climate sites ranked July 2023 among their 20 warmest Julys on record and nineteen climate sites ranked July among their 20 all-time warmest months on record. States like Virginia and Pennsylvania saw 100-year and 500-year storm events.

In August, temperatures dipped again with 29 out of the 35 major climate sites recorded as cooler than normal. We also saw 130 percent of normal precipitation within the region. During August, there were also 30 confirmed tornadoes in the Northeast, reaching seven times the region’s August average of four tornadoes (based on 1998-2022 data).

September ranked among the 20 warmest Septembers on record for 10 states and ranked among the 20 wettest on record for five states. September’s precipitation was highly variable, ranging from 71% of normal in West Virginia to 215% of normal in Connecticut. Two tropical systems affected the Northeast in September. Hurricane Lee produced rough surf and rip currents along the East Coast from around September 13 to 16 and Tropical Storm Ophelia resulted in multiple days of higher-than-usual water levels and contributed to major flash flooding in New York City on the 29th.

Data and summary adapted from https://www.nrcc.cornell.edu/

Maximum Hypoxic Area

CTDEEP utilizes Geographic Information System (GIS) software to create interpolations of bottom water dissolved oxygen concentrations across LIS and estimate the area effected by hypoxia on a cruise by cruise basis. A video was created to show changes in the maximum hypoxic area over time.

In 2023, the maximum area of Long Island Sound with bottom water dissolved oxygen (DO) concentrations below 3.0 mg/L was 126.8 sq. mi (328.4 sq. km) and occurred during the WQAUG23 survey conducted 31 July - 3 August 2023. During this survey four stations had DO concentrations below 2 mg/L, eight stations had concentrations below 3.0 mg/L, and 10 stations had concentrations between 3 and 3.5 mg/L. The lowest dissolved oxygen concentration recorded during the survey was 1.69 mg/L at Station A4.

The LISS utilizes a five-year rolling average to determine progress towards meeting the management target of reducing the area of LIS bottom water exhibiting hypoxic conditions. In assessing trends, LISS uses the five-year rolling average, instead of from year to year, because conditions in any given year could be impacted by variable factors, such as extreme changes in heat or precipitation, which average out over time.The years 1987-1999 are used as a benchmark (or the baseline) because they represent the beginning of Long Island Sound Study’s water quality monitoring program, prior to the Total Maximum Daily Load (TMDL) agreement in 2000 to reduce nitrogen loads into the Sound. Read more about the hypoxia extent ecosystem target established by the LISS.

This interactive graph shows the maximum area of hypoxia in square miles (blue bars), duration of hypoxic conditions in days (black circles), and the five-year rolling average area in square miles (green line).
Area in square miles, Duration in days

Duration

The duration of hypoxia refers to the estimated length of time that hypoxic conditions persist in the bottom waters of Long Island Sound. Duration is estimated based on dissolved oxygen concentration readings taken at three water quality monitoring stations in the Western Sound - A3, A4, and B3.

The 2023 hypoxic event lasted an estimated 42 days, beginning on July 12, 2023 and ending on August 22, 2023.

The earliest onset of hypoxia (based on CT DEEP data only) occurred on June 25, 2002 and the latest end date was September 20, 2005. The average duration over the 33-year time series is 52 days. The longest hypoxic event was 79 days during 2008. The shortest hypoxic event was 26 days in 2017.

Hypoxic Volume

Hypoxic Volume is a measure of the vertical extent (thickness) of hypoxia and is important from an ecological perspective for fish and crustaceans that move throughout the water column.

In 2019, CT DEEP and the O’Donnell lab at the University of Connecticut Marine Sciences Department undertook a project to develop a tool to calculate the hypoxic volume of Long Island Sound. The tool is available to the public and allows users to obtain area and volume estimates on the fly from any survey from 1991-present. The tool utilizes CT DEEP and IEC data. (NOTE: Area estimates from the tool will differ slightly from CTDEEP estimates due to slight differences in methodology.)

The maximum volume of water with concentrations below 3.0 mg/L occurred during the WQXXX23 survey and was XXX km3 (XXX mi2). The table compares hypoxic area and volume by survey over the 2023 season.
2023 Volume Estimates of Hypoxic Long Island Sound Bottom Waters Obtained from the UCONN HypVolApp
Area (sq km)
Volume (sq km)
CRUISE 2 mg/L 3 mg/L 4.8 mg/L 2 mg/L 3 mg/L 4.8 mg/L
WQJUN23 0 0 0 0.00 0.00 0.00
HYJUN23 0 0 0 0.00 0.00 0.00
WQJUL23 0 0 220 0.00 0.00 1.76
HYJUL23 0 0 416 0.00 0.00 11.20
WQAUG23 104 200 1104 0.51 1.82 38.05
HYAUG23 0 136 1168 0.00 1.22 37.60
WQSEP23 44 152 1092 0.00 0.54 32.29
HYSEP23 0 0 100 0.00 0.00 1.70

The interactive graph plots the maximum hypoxic volume over time. Initial analyses show “strong evidence that the hypoxic volume in LIS is substantially smaller between 2015 and 2019 than 20 years earlier” (O’Donnell, et al., 2020).

O’Donnell, J., Fake, T., and J. O’Donnell. 2020. Computing the Hypoxic Volume of Long Island Sound- Final Report, September 12, 2020. University of Connecticut, Department of Marine Sciences, Groton, CT. Prepared for the Connecticut Department of Energy and Environmental Protection and the Long Island Sound Study

Frequency of Hypoxia

This map illustrates how frequently the areas of Long Island Sound experience hypoxia. The colors represent the percentage of years in which hypoxic conditions have occurred in the bottom waters of Long Island Sound. The westernmost areas have experienced hypoxia almost every year since monitoring began.

The bar graph provides a deeper dive into the data (percentages) used to create the frequency map. The colors of the bars correspond to the map legend.